1 68 



STUDIES IN LUMINESCENCE. 



100 



90 



80 



70 

 ? 60 



tance of the flame in the other. The experiments were performed in a dark 

 room with an elaborate system of screens to prevent reflections, and 

 several independent tests convinced us that the inverse-square law of dis- 

 tances was very exactly satisfied. If the intensity is computed by the law 



of inverse squares, and if a 

 curve is plotted showing the 

 relation between intensity 

 and slit width, the results 

 obtained are of the type illus- 

 trated in Fig. 164. 



It will be seen that for nar- 

 row slits the intensity of the 

 transmitted light is not pro- 

 portional to the slit width. 

 When the width exceeds a 

 few hundredths of a milli- 

 meter the line becomes 

 straight, so that equal incre- 

 ments in slit width corre- 

 spond to equal increments in 

 intensity. The conditions, 

 whatever they are, which 

 lead to the curve in the 

 neighborhood of the zero of Fig. 164 are equivalent in their effect to a shift 

 in the zero point of the screw by about 2 divisions. The intensity trans- 

 mitted by a slit 50 divisions wide is not twice as great as that transmitted 

 by one 25 divisions wide, but the ratio is in reality 48 to 23. 



o 



s: 

 > 



5 



50 



40 



30 



20 - 



10 





10 



20 30 40 50 60 



Drum reading 

 Fig. 163. 



70 



80 90 100 



2 1 6 O /O /Z /-} 1/3 /& ZO 22 

 SLIT OP WAV OS IN HUNDREDTHS OP A MILLIf1TR 



i* 



Fig. 164. 



These experiments were not made with the same instrument that had 

 been used by ourselves and later by Miss Wick, but refer to an exactly 

 similar Lummer-Brodhun spectrophotometer. It seems probable that 

 this effect, due possibly to diffraction or to reflection from the jaws of the 



